The present invention relates to methods of storing information and, more particularly, to a novel method for storing data in an archival memory semiconductor target by means of melting apertures in a non-wetting material layer upon a surface of the target, and the target produced thereby.
Archival memory media, exhibiting a read-only structure, is often required for storing large quantities of data, usually in a digital binary format. The storage media must be non-volatile, whereby data is stored without changing content over long periods of time. Several methods have been proposed for writing binary data into small storage sites, typically having center-to-center spacings on the order of microns, to facilitate the necessary packing densities. Typically, digital information is initially entered into memory and maintained essentially in an unchanged state for the entire storage interval, once the authenticity and parity of the data written into the memory is confirmed. The data may be frequently accessed and read, whereby the same stored information is repeatedly retrieved.
It is known to fabricate archival memory media of a semiconductor target type, by the methods and as the structures disclosed and claimed in U.S. Pat. No. 4,064,495, issued Dec. 20, 1977; U.S. Pat. No. 4,081,794, issued Mar. 28, 1978; and U.S. Pat. No. 4,099,261, issued July 4, 1978, all assigned to the assignee of the present invention and all incorporated in their entirety herein by reference. Each of the aforementioned references utilizes a planar junction diode structure upon which binary-valued data is stored by modification of the semiconductor properties of at least one of a pair of semiconductor layers in a diode structure. Such modification of a semiconductor layer structure may be by means of ion-implanted diodes as disclosed and claimed in U.S. Pat. No. 4,064,495; by formation of P-N junction diodes alloyed into the surface of the chosen semiconductor layer, as disclosed and claimed in U.S. Pat. No. 4,081,794; or by damaging the semiconductor lattice of the chosen layer by accelerating ions for impingement upon the layer surface to induce damage therein to a controlled depth, as disclosed and claimed in U.S. Pat. No. 4,099,251. It is desirable to provide a semiconductor archival memory target in which a planar diode structure is unnecessary and which requires a minimal amount of energy to impinge thereon for writing an individual data bit into the target, whereby the impinging beam, typically being a beam of electrons, may write data into the target at an accelerated rate due to the reduced requirements for switching a high-energy beam between two energy values, each corresponding to either a first binary value or a second binary value.
It has been proposed that binary data be recorded on a target by evaporating off a thin layer of selenium, or tellerium, from the surface of an insulative layer supported by a semiconductor substrate, in selected areas where a data bit of a first binary value is to be recorded. A writing electron beam impinges upon the desired area for evaporation of the selenium; subsequently, a reading electron beam (of energy insufficient to evaporate the selenium) is directed through the hole evaporated in the selenium and enters the silicon-insulator sandwich with subsequent recovery of a first output level. Impingement of the reading electron beam upon an area in which a hole has not been evaporated, prevents electrons of the beam from entering the insulator-semiconductor sandwich and a provides a different value of target output current, to read the remaining binary data bit value. To achieve a difference in target output signals, between the pair of binary data values, the insulator layer is required to be sufficiently thin whereby spot heating of the selenium layer, for evaporation of apertures therein, induces heat energy sufficient to disrupt the integrity of at least the underlying insulative layer. The resulting target has reduced difference in signal output levels for the binary data values and may, in fact, be rendered unusable if sufficient heat energy is utilized during the data-writing evaporation step. It is therefore desirable to provide a method for recording binary data in an archival memory semiconductor target, in which reduced heating of the target occurs during the data writing step.